Devices for displaying information or identifications are constantly exposed to environmental effects, particularly in the form of pollution. This applies especially to the field of road traffic, and sooner or later impairs the legibility of information displayed. Particularly in the area of safety-relevant information, cleaning of the respective device becomes necessary, involving corresponding costs in the form of time and money. Such devices preferably should be self-cleaning to lighten the burden of the maintenance costs in question.
EP-B-0 772 514 discloses self-cleaning surfaces of objects having an artificial surface structure including elevations and depressions of a single type. The distance between elevations ranges from 5 to 200μ. The height of the elevations ranges from 5μ to 100μ. In addition, at least the elevations are made of water-repellent polymer materials or ones rendered water-repellent for protracted periods, and are not to be detachable by water or water containing detergents.
The disclosed surface has such elevations for repelling pollutants, a lotus leaf structure being imitated artificially. This structure is known to be self-cleaning in the sense that it is not contaminated. Even commercially available adhesives are repelled by the biological structure. Despite noteworthy results as regards self-cleaning effects, the disclosed surfaces may be employed only within limits, since either the area of materials to be used in manufacture is greatly restricted or the surface must undergo costly finishing operations related to hydrophobizing. Such surfaces are accordingly applied in facing design or as exterior wall paint. It has been found in practice, however, that the surfaces produced in this manner with the “lotus effect” often do not yield the desired results with respect to self-cleaning.
EP-A-0 933 388 discloses a structured surface possessing water-repellent and/or oil-repellent properties and low surface energy. These surfaces exhibit large wetting angles with water, with difficulty are wetted with water, and consequently, exert a self-cleaning effect. In order to accomplish such effect, an artificially producible base structure is provided with two different types of projections. One type of smaller projections are mounted on a superstructure, and are in the form of geometrically larger projections immediately adjacent to each other. To produce the disclosed projections and the superstructure as projections of another type, the latter are simultaneously or sequentially impressed into the surface material mechanically, etched in by lithographic processes, applied by a shaping process or obtained by casting technology. In the mechanical impressing process, force is suitably applied from the rear side toward the surface. The two types of structural elevations on its opposite side are then formed. The casting, impressing, etching, and application processes used for this purpose are not suited for industrial-scale production of large quantities of structured surfaces, even though this disclosed system yields very good results in the case of self-cleaning, which incidentally has its counterpart in nature in the form of the leaf surface of the nasturtium.